1. Technical Field of the Invention
The present invention relates to channel tracking, and more particularly, to an improved method of channel tracking involving only the medium response of a received signal.
2. Description of Related Art
A communication system is formed, at a minimum, of a transmitter and a receiver interconnected by a communication channel. The communication system is operable at least to transmit communication signals, having an informational content, generated at, or applied to, the transmitter upon the communication channel. The receiver is operable to receive the transmitted, communication signal and to recreate the informational content of the communication signal.
A radio communication system is a communication system in which the communication channel is formed of one or more frequency bands of the electromagnetic frequency spectrum. A transmitter operable in a radio communication system generates a communication signal of characteristics permitting its transmission upon the communication channel, and the receiver operable in the radio communication system is operable to receive the communication signal transmitted upon the communication channel.
Typically, the radio receiver includes tuning circuitry tunable to the frequencies of the communication channel upon which the communication signal is transmitted, down-conversion circuitry for down-converting a receive signal formed of the communication signal from a transmission frequency into a lower frequency, or baseband signal, demodulation and decoder circuitry which permits the informational content of the communication signal to be recreated. A radio communication system is advantageous for the reason that a fixed, or hard-wired, connection is not required to form the communication channel extending between the transmitter and receiver. Communication can be effectuated between remotely-positioned transmitters and receivers without the need to form the hard-wired or other fixed connection therebetween.
Technological advancements in communication technologies have permitted communication systems to utilize digital communication techniques. Some existing communication systems have been converted to permit the utilization of digital communication techniques; other communication systems have been planned, or have been made possible, as a result of such technological advancements. Utilization of digital communication techniques is advantageous as information can be more efficiently transmitted upon a communication channel than by utilizing conventional, analog communication techniques. Also, some transmission difficulties which distort the communication signal as the communication signal is transmitted by a transmitter to a receiver can sometimes be more readily overcome when digital communication techniques are utilized.
A transmitter operable in such a communication system digitizes an information signal to form a digital signal. Once digitized, the digital signal can be modulated by a digital modulation technique, such as by a Gaussian Minimum Shift Keying (GMSK) modulation technique. The digitized signal, once modulated, is transmitted upon a communication channel.
A receiver operable to receive the signal transmitted upon communication channel includes circuitry to demodulate the signal received and to form a digitized signal which can then be converted into another form, if desired, such as by a decoding process.
Distortion is sometimes introduced upon the signal transmitted by the transmitter. The distortion can, for instance, be caused by filter circuitry of the transmitter, or filter circuitry of the receiver, or the communication channel. The filter circuitry of the transmitter, the communication channel, and the filter circuitry of the receiver shall, at times, be together referred to as the "composite channel" below.
One type of distortion which is sometimes introduced on the signal is referred to as intersymbol interference. The causes of intersymbol interference and the distortion caused therefrom is well-known.
Receivers operable to receive signals in a digital communication system sometimes include circuitry, referred to as channel estimator circuitry, which estimates the channel characteristics, namely, the channel impulse response, of the composite channel. Such channel estimators generate an estimate of a channel impulse response of the composite channel. The channel impulse response estimated by the channel estimator is used by receiver equalizer circuitry to counteract the intersymbol interference, thereby to permit the receiver to recreate more accurately the informational content of the communication signal actually formed at the transmitter.
The quality of the channel estimate is important since an accurate estimation of the channel impulse response is determinative of the ability of the receiver equalizer to counteract for intersymbol interference. The incorporation of channel tracking within an equalizer has been used when the tracked channels vary fast enough over the duration of the data being equalized. This approach is used in a D-AMPS equalizer at both the base station and the mobile station. Most existing channel tracking systems track the overall response of a channel, which consists of the medium response plus the fixed filter response. Existing systems also normally utilize symbol space sampling for processing the received signal.